In most magnetic recording materials presently used, thermoplastic resins, such as vinyl chloride-vinyl acetate copolymer resins, vinyl chloride-vinylidene chloride copolymer resins, cellulose resins, acetal resins, urethane resins, acrylonitrile-butadiene resins and the like, are used as binders independently or in combination of two or more thereof. However, these materials suffer from the defect that when used for a magnetic tape parts of the magnetic layer may be removed and contaminate the running passage of the magnetic tape such as guide posts, magnetic heads, cylinders, etc. due to inferior abrasion resistances of their magnetic layers.
In addition to the above materials, thermosetting resins such as melamine resins, urea resins and the like, or chemically cross-linkable binders obtained by adding cross-linking agents, such as isocyanate compounds, epoxy compounds and the like, have also been used as binders for magnetic tapes. However, the use of cross-linkable binders causes two serious disadvantages: (1) a resinous solution in which a magnetic substance has been dispersed has a low storage stability, that is, a short pot life, and, therefore, the homogeneity of the physical properties of the magnetic coating composition cannot be maintained which, in turn, deteriorates the uniformity of the magnetic tape; and (2) after coating and drying the magnetic coating composition, the coated composition must be subjected to heat treatment for hardening the coated layer whereby the process requires a prolonged time to obtain the final magnetic recording materials.
With the intention of obviating the above-described disadvantages, methods of producing magnetic materials in which mixtures of the oligomers and the monomers of acrylic acid ester type are used as binders, and their magnetic layers are hardened by irradiation with electron beams are disclosed in Japanese Patent Publication No. 12423/72; Japanese Patent Application (OPI) Nos. 15104/72 (French Pat. No. 2,120,184 and German Pat. Nos. 2,100,037), 77433/75 and 25235/81, etc; and U.S. Pat. Nos. 3,104,983 and 4,004,997. These prior art references disclose the use of magnetic recording composition hardenable by irradiation with electron beams, but do not teach a subbing layer or surface treatment of supports on which the magnetic recording layer is provided. These conventional magnetic recording materials have, therefore, disadvantages that the magnetic recording layer tends to be peeled off during repeated use due to insufficient adhesion between the magnetic recording layer and the support.
In recent years, a demand for long time recording and light-weight magnetic tapes has increased. In order to meet this demand, it is indispensable to use a thin support. Further, the magnetic recording material must be usable under conditions which are much more severe than before because recent record-reproducing apparatuses, such as video tape recorder, audio tape recorder, etc., have various functions. According to conventional methods using a hardening reaction caused by irradiation with electron beams, magnetic tapes obtained create various troubles, for example, the magnetic tape may be broken during the running operation, the magnetic layer may peel off in a recorder and contaminate the running course of the magnetic tape, or the magnetic layer may peel off during a smoothening procedure using a calender. Therefore, magnetic recording materials having excellent properties have not yet been obtained.
As a result of various studies with the intention of solving the above-described problems making the most of advantages of an electron-beam hardenable binder, the present inventors completed the present invention.